Re: [PATCH] fuse: dax: No-op writepages callback

[Asahi Lina]

On 11/13/24 7:48 PM, Miklos Szeredi wrote:
> On Tue, 12 Nov 2024 at 20:55, Asahi Lina <lina@xxxxxxxxxxxxx> wrote:
>>
>> When using FUSE DAX with virtiofs, cache coherency is managed by the
>> host. Disk persistence is handled via fsync() and friends, which are
>> passed directly via the FUSE layer to the host. Therefore, there's no
>> need to do dax_writeback_mapping_range(). All that ends up doing is a
>> cache flush operation, which is not caught by KVM and doesn't do much,
>> since the host and guest are already cache-coherent.
> 
> The conclusion seems convincing.  But adding Vivek, who originally
> added this in commit 9483e7d5809a ("virtiofs: define dax address space
> operations").
> 
> What I'm not clearly seeing is how virtually aliased CPU caches
> interact with this.  In mm/filemap.c I see the flush_dcache_folio()
> calls which deal with the kernel mapping of a page being in a
> different cacheline as the user mapping.  How does that work in the
> virt environment?
> 

Oof, I forgot those architectures existed...

The only architecture that has both a KVM implementation and selects
ARCH_HAS_CPU_CACHE_ALIASING is mips. Is it possible that no MIPS
implementations with virtualization also have cache aliasing, and we can
just not care about this?

~~ Lina